The adaptation of this novel is so much different from the Japanese movie and from the novel itself and is more bonkers then the American adaptation. I will try my best with this convoluted story line. It does play into Japanese tropes of Lolita complex and repression of outward feelings.
The mother persona never leaves the daughter body like in the American adaptation and the mother takes it as her chance to have a second chance at life and do things should didn't do in her previous life.
The mother persona in the daughters starts dating the son of the bus driver that caused their car accident and end up dating him!!
Knowing that their relationship wont work with her husband while living in her daughter's body. She fakes a split personality disorder and pretends her daughters persona is coming back and taking back the daughters body so she can then date said son and marry him.
The husband realizes that the mother persona never left and was faking the daughter persona so they can both move on with their different lives. This becomes the secret they keep not telling the other what the other one knows.
Wikipedia did it better.
https://en.wikipedia.org/wiki/Naoko_(novel)
I love the author Keigo Higashino, since he is one of the few English translated works I could read while I was living in Niigata, Japan. he is a really great murder mystery novelist (Devotion of Suspect X is really good and won many awards and I recommend reading or listening on Audible) and the book The Secret is based on Naoko is based on Japanese surrealism like any Murakami novel. Thanks Paul, June and Jason, I have been listening to you since Burlesque and it helped when I was living in Japan during the 3/11 quake .